Theory:

The pattern of rhymes at the ending of each line in a poem is called a rhyme scheme. Letters (A,B,C...) are usually used to express which lines rhyme. Verses that are designated with the same letter are said to rhyme with each other. It is also known as an arrangement of rhymes in a stanza or a poem.
Example:
For easier understanding of the concept, let us take a famous nursery rhyme, written by Jane Taylor as an example.
 
Twinkle twinkle little star,
How I wonder what you are.
Up above the world so high
Like a diamond in the sky.

We see that the first two lines rhyme with each other (star-are); the second two lines rhyme with each other (high-sky). Let us name each line as  \(A\), \(B\)  depending on the words that rhyme with each other.
 
Twinkle twinkle little star,\(A\)
How I wonder what you are.\(A\)
Up above the world so highB
Like a diamond in the sky.B
 
It can thus be seen that this poem follows \(AABB\) pattern.
Rhyme scheme of the poem" The Kite":
How bright on the blue
A
Is a kite when it’s new!A
  
With a dive and a dipB
It snaps its tailC
Then soars like a shipB
With only a sailC
  
As over tidesA
Of wind it rides,A
  
Climbs to the crestB
Of a gust and pulls,C
Then seems to restB
As wind falls.C
  
When string goes slackA
You wind it backA
  
And run untilB
A new breeze blowsC
And its wings fillB
And up it goes!C
  
How bright on the blueA
Is a kite when it’s new!A
  
But a raggeder thingB
You never will seeC
When it flaps on a stringB
In the top of a tree.C
Important!
It can thus be seen that this poem follows \(AABCBC\) pattern.
Reference:
National Council of Educational Research and Training (2006). Honeysuckle. The Kite  (pp. 27). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.